Infrared-transmitting, polarization-maintaining optical fiber and method for making

ABSTRACT

This application relates generally to an optical fiber for the delivery of infrared light where the polarization state of the light entering the fiber is preserved upon exiting the fiber and the related methods for making thereof. The optical fiber has a wavelength between about 0.9 μm and 15 μm, comprises at least one infrared-transmitting glass, and has a polarization-maintaining (PM) transverse cross-sectional structure. The infrared-transmitting, polarization-maintaining (IR-PM) optical fiber has a birefringence greater than 10−5 and has applications in dual-use technologies including laser power delivery, sensing and imaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of U.S. ProvisionalApplication No. 62/830,732, filed on Apr. 8, 2019, the content of whichis incorporated herein by reference in its entirety.

TECHNICAL FIELD

This application relates generally to an optical fiber for the deliveryof infrared light where the polarization state of the light entering thefiber is preserved upon exiting the fiber and the related method formaking. The optical fiber has a range of transmissible wavelengthsbetween about 0.9 μm and 15 μm, comprises at least oneinfrared-transmitting glass, and has a polarization-maintaining (PM)transverse cross-sectional structure.

BACKGROUND OF THE INVENTION

Infrared (IR) fibers are used in many passive and active applications totransmit, generate, or modify infrared optical energy at wavelengthsfrom as low as 0.9 μm to as high as 20 μm. Infrared fibers typicallyhave axial symmetry which is agnostic with respect to the linearpolarization state of the light transmitted therein. IR fibers,typically comprise a solid core region surrounded by a solid claddingregion, usually surrounded by a protective coating. Solid-core IR fibersare made from IR-transmitting materials including chalcogenide glasses,fluoride glasses, and crystalline materials such as silver halides,thallium bromoiodide (KRS-5), sapphire, and yttrium aluminum garnet(YAG). This is in contrast to the more common silica glass opticalfibers that transmit visible light and some near-infrared (NIR) lightincluding wavelengths as low as about 400 nm to as high as about 1.8 μm.

Axially asymmetric optical fibers for visible wavelengths have beendesigned with physical features that impart modal birefringence suchthat orthogonal polarizations have different propagation constants andthere is low mode-coupling or crosstalk between them. Many types of PMfibers have been demonstrated for visible and near-infrared (NIR)telecommunications wavelengths (400 nm-1800 nm), largely built uponsilica glass with various dopants. However, no PM fiber has beendeveloped for infrared wavelengths (2-20 μm), and there have been veryfew IR-PM fibers reported in the scientific literature and none arecommercially available. The typical IR application space: laser powerdelivery, low data rate communications, chemical sensing and others, hasnot traditionally required these types of fibers and little effort hasgone into their development beyond technological curiosity. It is worthnoting that solid-core silica fibers do not transmit IR light wellbeyond about 2 μm due to the infrared absorption of silica, and PMsilica fibers cannot be used at the wavelengths of interest (2-20 μm) inthis invention. Through analysis of thermal stress in silica PM fibers,Chu et al. derived functional forms for the resultant stress and thematerial birefringence due to the materials and geometry in commonstress-induced birefringent silica-based PM fibers (Chu et al.,“Analytical method for calculation of stresses and materialbirefringence in polarization-maintaining optical fiber,” J. LightwaveTechnol., vol. 2, no. 5, pp. 650-662 (1984)). They approximate thebirefringence as

$\begin{matrix}{B \cong {\frac{2EC}{1 - v}( {\alpha_{2} - \alpha_{3}} ){{T( \frac{d_{1}}{d_{2}} )}^{2}\lbrack {1 - {3( {1 - {2( \frac{r}{b} )^{2}}} )( \frac{d_{2}}{b} )^{4}} + {3( \frac{r}{d_{2}} )^{2}\cos 2\theta}} \rbrack}}} & ( {{Eq}.\mspace{14mu} 1} )\end{matrix}$

where E is Young's modulus of the glass, C is the stress-opticcoefficient of the glass, v is Poisson's ratio of the glass, α₂ is thethermal expansion coefficient of the cladding glass, α₃ is the thermalexpansion coefficient of the stressor glass, T is the temperaturedifference between the fiber drawing and room temperatures, d₁ is thehalf diameter or radius of each circular stressor member, d₂ is thedistance from the center of each stressor member and the center of thefiber core, b is the half diameter or radius of the fiber cladding and rand θ are the usual radial coordinates. The magnitude of thebirefringence is therefore strongly dependent on the materials availableto the fiber engineer and the fiber drawing temperature, which forsilica-based fibers is very large.

The fibers of the present invention comprise non-silica-containingglasses including chalcogenide glasses, comprising at least onechalcogen element excluding oxygen (S, Se, Te); chalcohalide glasses,comprising at least one chalcogen element excluding oxygen (S, Se, Te)and at least one halogen element (F, Cl, Br, I); fluoride glasses,comprising fluorine and at least one other element (e.g. ZBLAN); andheavy-metal oxide glasses comprising oxygen and at least one metalelement excluding silicon (e.g. GeO₂—TeO₂). It should be apparent to oneskilled in the art of fiber optics and specifically the art of infraredfiber optics that one cannot simply replace silica with a more suitableIR-transmitting glass to make an IR-PM fiber as the material properties(Young's modulus, stress-optic coefficient, Poisson's ratio, thermalexpansion coefficient, softening temperature and others) needed to do soare not available in commercially available infrared glasses and thebirefringence attainable using such glasses is therefore very low andnot usable for IR-PM fiber. For example, the softening temperature ofinfrared glasses in general and chalcogenide glasses more specificallyis very low (200-400° C.) in comparison to silica which is drawn intofiber at 1900° C. The T term in Equation 1 above is therefore about 5 to10 times greater for silica fibers than chalcogenide fibers. The Young'smodulus of silica is about 5 times greater than that of chalcogenideglasses. Silica has a very low thermal expansion coefficient but it ispossible to increase the thermal expansion with the addition of otherelements as dopants to be used as stressor members resulting in a large(α₂-α₃) term for silica PM fibers. Chalcogenide glasses typically have athermal expansion coefficient about 40 times larger than silica and,unlike silica, even small chemistry changes can greatly impact thesoftening temperature making modification of α₃ through dopingimpractical in a PM fiber based on chalcogenide glasses. Thus the(α₂-α₃) term for a silica PM fiber is about 2 to 5 times larger than fora hypothetical chalcogenide PM fiber. Combined the maximum birefringencefor a PM silica fiber is about 50 to 250 times larger than for ahypothetical chalcogenide PM fiber of the same design. Additionally,chalcogenide fibers become quite weak when stressed reducing thepractical birefringence of a chalcogenide glass PM fiber even lower.

Infrared (IR) transmitting fibers are commonplace in specialtyapplications and can be solid-core or hollow-core. Solid-core IR fibersare made from IR-transmitting materials including chalcogenide glasses,fluoride glasses, and crystalline materials such as silver halides,thallium bromoiodide (KRS-5), sapphire, and yttrium aluminum garnet(YAG). Solid-core IR fibers can have a structure similar to core-cladfibers used in visible-NIR applications, with a solid core surrounded bya solid cladding that is surrounded by a protective material (often aphoto-cured or thermal-cured polymer), but can also have amicro-structured cladding wherein the core is surrounded by a series ofholes or voids disposed within a second solid material, that can be thesame or similar to the core material. The second type is often called amicro-structured optical fiber (MOF) or a photonic crystal fiber (PCF)if the holes are positioned in some regular arrangement. The arrangementof voids in a MOF can be used to impart birefringence in the fiber(Ortigosa-Blanch et al., “Highly birefringent photonic crystal fibers,”Opt. Lett., vol. 25, no. 18, pp. 1325-1327 (2000)) and has beendemonstrated in fibers made from an IR glass (Caillaud et al., “Highlybirefringent chalcogenide optical fiber for polarization-maintaining inthe 3-8.5 μm mid-IR window,” Opt. Exp., vol. 24, no. 8, pp. 7977-7986(2016)). The terms MOF and PCF are often used interchangeably, eventhough PCF connotes a precise ordered arrangement of optical elements inthe fiber and is more technically a subset or type of MOF.

Solid-core IR-transmitting fibers made from chalcogenide glasses aremost often formed using one of two methods: a preform drawing process ora double-crucible process (DCP). Fibers made from DCP have lower opticallosses due to precision geometry (minimizes waveguide losses) and verylow impurities (minimizes extrinsic material absorptions). A doublecrucible is a device comprising a pair of concentric crucibles, calledthe core crucible and clad crucible, each connected to its own hopperfor supplying glass, each having an exit aperture or tip with asubstantially circular transverse cross-sectional shape. The doublecrucible fiber drawing process is a method comprising loading a coreglass into the core glass hopper, loading a cladding glass into the cladglass hopper, heating the glasses to soften them and transfer at leastsome of each glass to its crucible, substantially filling bothcrucibles, introducing an inert gas to each sealed hopper at controlledpressures and rates such that core and cladding glass exit the cruciblesat their tips and contact each other without any gap between them andthe glass is continuously pulled from the crucible forming a fiberhaving a core comprising the core glass and a clad comprising thecladding glass. When the fiber exiting the double crucible process hasan outer diameter larger than about 0.5 mm it is often called a canebecause it is typically less flexible than a fiber. Canes can be used inwaveguiding applications or used as components in another assembly thatmay be drawn into fiber including but not limited to MOF.

In hollow-core IR fibers, light propagates within a hollow voidsurrounded by one or more materials that confine the light to the hollowcore. The confining material can be a metal such as silver, often with areflection enhancing dielectric coating applied to it, or a dielectricthat has anomalous dispersion (refractive index less than unity) for thewavelength range of interest (such as sapphire at 10.6 μm), or adielectric with a series of holes or voids disposed therein such thatthe voids form a periodic structure with a photonic band gap (PBG) atthe wavelength range of interest. In the PBG fiber, light is confined toa hollow core by virtue of the surrounding photonic crystal structurehaving a bandgap corresponding to the frequency of the light beingtransmitted, such that light cannot propagate through the PBG cladding.More recently, another class of hollow MOF, the so-called inhibitedcoupling (IC) fiber has been the subject of much research (Argyros etal., “Antiresonant reflection and inhibited coupling in hollow-coresquare lattice optical fibres,” Opt. Exp., vol. 16, no. 8, pp. 5642-7(2008) and Wei et al., “Negative curvature fibers,” Adv. Opt. Photon.,vol. 9, no. 3, pp. 504-561 (2017)). IC fiber does not confine light tothe hollow core using a bandgap, but does so by inhibiting coupling ofthe electromagnetic modes of the fiber core and those within the glasscomprising the cladding. Typically, IC fibers have a plurality ofthin-walled round tube members arranged in a circle and evenly spacedsuch that they are not touching each other, but are touching a commonsupport tube surrounding them. The void formed at the center of thesemembers comprises the fiber core having a boundary with a complex ornegative curvature. Because of this feature, these fibers are alsocalled negative curvature fibers (NCF) (Belardi et al., “Effect of coreboundary curvature on the confinement losses of hollow antiresonantfibers,” Opt. Exp., vol. 21, no. 19, p. 21912 (2013) and Gattass et al.,“Infrared glass-based negative-curvature anti-resonant fibers fabricatedthrough extrusion,” Opt. Exp., vol. 24, no. 22, pp. 25697-25703 (2016)).Other names commonly used are frustrated or anti-resonant fibers (ARFs)and tube lattice fibers (TLFs) (Setti et al., “Flexible tube latticefibers for terahertz applications,” Opt. Exp., vol. 21, no. 3, pp.3388-3399 (2013)). Anti-resonant fibers can be designed withbirefringence for PM applications although none have been demonstrated(Wei et al., “Polarization-filtering and polarization-maintaininglow-loss negative curvature fibers,” Opt. Exp., vol. 26, no. 8, pp.9528-13 (2018); Vincetti et al., “Elliptical hollow core tube latticefibers for terahertz applications,” Optical Fiber Technology, vol. 19,no. 1, pp. 31-34 (2013); and Mousavi et al., “Broadband highbirefringence and polarizing hollow core antiresonant fibers,” Opt.Exp., vol. 24, no. 20, pp. 22943-16 (2016)).

Caillaud et al. demonstrated polarization-maintaining chalcogenide glassmicro-structured optical fiber (MOF) for 3-8.5 μm in the infrared(Caillaud et al., “Highly birefringent chalcogenide optical fiber forpolarization-maintaining in the 3-8.5 μm mid-IR window,” Opt. Exp., vol.24, no. 8, pp. 7977-7986 (2016)). Their fiber comprises a solid coresurrounded by 3 rings of air-holes with diameter, d=7.64 μm, arranged ina hexagonal lattice having a lattice pitch, Λ, disposed within a solidAs—Se based glass such that the hole-to-pitch ratio, d/Λ, is 0.45, andhaving a pair of larger holes with diameter, dh, adjacent to and onopposing sides of the core, such that the hole to pitch ratio for thelarger holes, dh/Λ, is 0.84. In this example, the authors utilize amolten glass casting method wherein the As—Se glass is melted to lowviscosity (10⁻⁴ PaS) and cast into a mold comprising a lattice of thinwalled silica capillaries. After casting, the silica capillaries areremoved by acid etching. This method is not compatible with otherchalcogenide glasses because they have strong adhesion to silica andfracture during cooling (Coulombier et al., “Casting method forproducing low-loss chalcogenide microstructured optical fibers,” Opt.Exp., vol. 18, no. 9, pp. 9107-9112 (2010)).

Gao et al. show a ZBLAN fiber with elliptical core and extinction ratioof about 6.7 dB over 10 m (Gao et al., “Third-harmonic generation in anelliptical-core ZBLAN fluoride fiber,” Opt. Lett., vol. 38, no. 14, pp.2566-3 (2013)).

Yao et al. demonstrate TeO₂—BaF₂—Y₂O₃ wagon wheel, suspended core fiberwhere the core is elliptical and therefore birefringent (Yao et al.,“Mid-infrared dispersive waves generation in a birefringentfluorotellurite microstructured fiber,” Appl. Phys. Lett., vol. 109, no.10, pp. 101102-6 (2016)). Although the authors do not show thetransmission spectrum for the fiber, fluorotellurite fibers may transmitvisible and MWIR 0.4-6 μm depending on the composition and impuritiespresent.

Gibson et al. demonstrate polarization-maintaining infrared air-corewaveguide with a rectangular bore (225×1250 μm) with a length of 125 cmtransmitting 10.6 μm light (Gibson et al., “Polarization-maintaininghollow glass waveguides with noncircular bore,” Opt. Eng., vol. 43, p.568 (2004)). The air core was surrounded by a silver-iodide coatedsilver mirror that was supported by borosilicate glass tubing substrate.

Sharma et al. propose but do not demonstrate a birefringent IR PCF basedon ZBLAN (Sharma et al., “Wavelength-tunable mid-infrared femtosecondRaman solution generation in birefringent ZBLAN photonic crystal fiber,”Journal of Modern Optics (2016)). The PCF comprises a square lattice ofair holes disposed within a ZBLAN glass fiber, the central core is solidZBLAN, and a pair of larger air holes adjacent to and on opposing sidesof the solid core.

Sultana et al. propose designs for, but do not demonstrate, highlybirefringent PCF for terahertz comprising a polymer (Topas) and ahexagonal lattice of air-holes surrounding a core (Sultana et al.,“Highly birefringent elliptical core photonic crystal fiber forterahertz application,” Optics Communications, vol. 407, pp. 92-96(2018)). The core is solid polymer with five elliptical air holes, eachspanning the entire core and evenly spaced with their fast axesco-aligned. The high-aspect ratio air holes within the core impartbirefringence in this design.

Dabas et al. propose but do not demonstrate a chalcogenide glass PCFwith high birefringence (Dabas et al., “Design of highly birefringentchalcogenide glass PCF: A simplest design,” Optics Communications, vol.284, no. 5, pp. 1186-1191 (2011)). Their fiber design comprises a solidcore surrounded by seven rings of air holes each having a diameter, d,arranged in a hexagonal lattice having a lattice pitch, Λ, disposedwithin a solid As₂Se₃ based glass such that the hole-to-pitch ratio,d/Λ, is controlled and having a pair of larger holes with diameter, d₂,in the first ring and adjacent to and on opposing sides of the core,such that the lattice positions of the larger holes and the coreposition are in a line forming a transverse axis, as well as a secondpair of smaller holes with diameter d₁, in the second ring around thecore such that the lattice positions of the smaller holes and the coreposition are in a line forming an second axis orthogonal to thetransverse axis and d₁<d<d₂. They disclose a fiber optimized for largebirefringence having d=1.1 μm, d₁=0.45 μm, d₂=2.4 μm and Λ=2.2 μm.

Zhang et al. propose but do not demonstrate a mid-infrared transmittingbirefringent PCF with chalcogenide glass, 65Gα₂S₃-32La₂S₃-3La₂O₃ (GLS)comprising a series of air holes having diameter do arranged on ahexagonal lattice with pitch Λ, having a solid high-aspect ratio coreoccupying three lattice positions in the center of the fiber, a pair ofsmaller holes adjacent to the core coincident with the core long axisand having diameter d_(c), and a number of larger air holes occupyingthe lattice positions along the and having diameter d_(v) (Zhang et al.,“Properties of high birefringence chalcogenide glass holey fibre formid-infrared transparency,” J. Opt., vol. 12, no. 3, pp. 035207-8(2010)).

SUMMARY OF THE INVENTION

The invention described herein, including the various aspects and/orembodiments thereof, meets the unmet needs of the art, as well asothers, by providing an infrared-transmitting, polarization-maintaining(IR-PM) optical fiber and methods for making thereof. In one embodiment,the fiber is made by forming a cane comprising an infrared-transmitting,non-silica glass, compressing the cane so that it has an approximatelyelliptical transverse cross-section and core, inserting the compressedcane into a tube to form an assembly, and stretching the assembly into afiber that is approximately round with an approximately elliptical corewhere the fiber has a birefringence greater than 10⁻⁵.

In one embodiment, the fiber is made by making a preform with anapproximately round cross-sectional shape comprising an approximatelyround core surrounded by a cladding where the preform comprises aninfrared-transmitting, non-silica glass, compressing the preform so boththe core and cladding have an approximately elliptical shape, alteringthe compressed preform by grinding, polishing, and/or machining to forman approximately round cladding, and drawing the altered preform into afiber that is approximately round with an approximately elliptical corewhere the fiber has a birefringence greater than 10⁻⁵.

In one embodiment, the fiber is made by making a preform with a corehaving an approximately circular cross-sectional shape and comprising afirst infrared-transmitting, non-silica glass and a cladding surroundingthe core where the cladding comprises a second infrared-transmitting,non-silica glass and has a cross-sectional shape that is not a circle.The preform is drawn into a fiber that is approximately round with anapproximately elliptical core where the fiber has a birefringencegreater than 10⁻⁵. During the drawing step, the preform is heldvertically and heated in a fiber drawing oven until one end of thepreform slumps and forms a tapered section and a drop, and then the dropis pulled at a controlled rate. The temperature of the preform in thetapered section is between 50 and 150° C. greater than the glasstransition temperature of the preform glass, and the viscosity of thepreform glass is 10⁴-10⁶ Poise.

In one embodiment, the fiber is made by providing a double cruciblehaving a core crucible and a clad crucible where at least one of thecrucibles has an approximately elliptical shaped cross-sectional exitaperture, loading a first infrared-transmitting, non-silica glass intothe core crucible, loading a second infrared-transmitting, non-silicaglass into the clad crucible, forming a glass with a core surrounded bya cladding, and drawing a fiber that is approximately round with anapproximately elliptical core where the fiber has a birefringencegreater than 10⁻⁴.

In one embodiment, the fiber is made by making a core cane comprising afirst infrared-transmitting, non-silica glass, making a cladding preformcomprising a second infrared-transmitting, non-silica glass where thecladding preform comprises a plurality of openings, making at least twostressor canes comprising a third infrared-transmitting non-silicaglass, forming an assemblage comprising the core cane and stressor canesinserted into the openings in the cladding preform, and stretching theassemblage into a fiber where the fiber has a birefringence greater than10⁻⁴.

In one embodiment, the fiber is made by making a preform comprising aninfrared-transmitting, non-silica glass where the preform has a solidcore and a plurality of openings and drawing the preform into a fibercomprising a solid care and a series of longitudinal air-holes arrangedin a lattice where the fiber has a birefringence greater than 10⁻⁴.

This invention provides IR-PM fibers that are not available elsewhere.IR-PM fibers have applications in dual-use technologies including laserpower delivery, sensing and imaging. Notable applications of interestinclude routing of high power laser energy for infrared countermeasuresand directed energy.

Other features and advantages of the present invention will becomeapparent to those skilled in the art upon examination of the followingor upon learning by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1H show an elliptical core polarization maintaining (PM) fiberand the cane or preform thereof. FIG. 1A shows a cane being compressedto form an elliptical transverse cross-section. FIG. 1B shows acompressed cane inside a jacket tube with a round hole. FIG. 1C shows acompressed cane inside a jacket tube with an elliptical hole. FIG. 1Dshows a fiber formed by stretching the cane and tube assembly on a fiberoptic drawing tower. FIG. 1E shows a core-clad preform with a roundtransverse cross-sectional shape. FIG. 1F shows the preform of FIG. 1Eafter being compressed perpendicular to the preform axis to deform thecross-sectional shape of both the core and cladding into an ellipticalshape. FIG. 1G shows the preform of FIG. 1F after the cross-sectionalshape of the cladding is made round by grinding, polishing and/ormachining. FIG. 1H shows the fiber drawn from the preform shown in FIG.1G in a manner consistent with rod-in-tube preform drawing.

FIGS. 2A-2D show an IR-PM preform and fiber with an elliptical coredriven by surface tension. FIG. 2A shows a preform comprising a corehaving a circular transverse cross-sectional shape surrounded by acladding having a transverse cross-sectional shape that is elliptical.FIG. 2B shows an IR-PM fiber having an elliptical core fabricated bystretching the preform shown in FIG. 2A on a fiber optic draw tower.FIG. 2C shows a preform comprising a core having a circular transversecross-sectional shape surrounded by a cladding having a transversecross-sectional shape that is approximately circular but with twotruncated or flattened sides. FIG. 2D shows an IR-PM fiber having anelliptical core fabricated by stretching the preform shown in FIG. 2C ona fiber optic draw tower.

FIGS. 3A-3F show an elliptical core IR-PM fiber drawn using the doublecrucible process (DCP) and the tip shape and glass exiting tip thereof.FIG. 3A shows the tip shape of a core crucible having a round transverseshape and a clad crucible having an elliptical transverse shape. FIG. 3Bshows a glass upon exiting the tip to provide a round shape to the fibercore glass and an elliptical shape to the fiber clad glass upon exitfrom the crucible tip. FIG. 3C shows an IR-PM fiber having a solid corewith an elliptical transverse cross-sectional shape drawn using the DCPwith the glass exiting tip shown in FIG. 3B. FIG. 3D shows the tip shapeof a core crucible having an elliptical transverse shape and a cladcrucible having a round transverse shape. FIG. 3E shows a glass uponexiting the tip to provide an elliptical shape to the fiber core glassand a round shape to the fiber clad glass upon exit from the crucibletip. FIG. 3F shows an IR-PM fiber having a solid core with an ellipticaltransverse cross-sectional shape drawn using the DCP with the glassexiting tip shown in FIG. 3E.

FIGS. 4A-4F show an IR-PM fiber with stress-induced birefringence andelements thereof. FIG. 4A shows a core cane. FIG. 4B shows a claddingpreform having multiple openings. FIG. 4C shows a pair of stressorcanes. FIG. 4D shows an assemblage comprising the core cane and stressorcanes inserted into the cladding preform. FIG. 4E shows an IR-PM fiberthat was stretched from the assemblage shown in FIG. 4D. FIG. 4F shows acore cane having an outer coating.

FIGS. 5A-5E show an IR-PM micro-structured optical fiber (MOF) and thepreform thereof. FIG. 5A shows a preform having a solid core comprisinga series of longitudinal air-holes arranged in a lattice surrounding thecore. FIG. 5B shows an IR-PM MOF where the core is birefringent by meansof a pair of air-holes on either side of the core having a diameterlarger than the diameter of the rest of the holes. FIG. 5C shows anIR-PM MOF where the core is birefringent by means of all holes on eitherside of and colinear with the core having a diameter larger than thediameter of the rest of the holes. FIG. 5D shows an IR-PM MOF where thecore is birefringent by means of a second IR-transmitting glass having arefractive index lower than the first IR-transmitting glass taking theplace of a pair of air holes on either side of the core. FIG. 5E showsan IR-PM MOF where birefringence is provided by means of substitution ofthe IR-transmitting glass in place of two air holes adjacent to andcolinear to the core.

FIGS. 6A-6F show an IR-PM solid MOF and the preform thereof. FIG. 6Ashows a preform comprising a first IR-transmitting glass having a solidcore comprising a series of longitudinal solid rods comprising a secondIR-transmitting glass arranged in a lattice surrounding the core. FIG.6B shows an IR-PM solid MOF where the core is birefringent by means of apair of rods on either side of the core having a diameter larger thanthe diameter of the rest of the rods. FIG. 6C shows an IR-PM solid MOFwhere the core is birefringent by means of all rods on either side ofand colinear with the core having a diameter larger than the diameter ofthe rest of the rods. FIG. 6D shows an IR-PM solid MOF where the core isbirefringent by means of a third IR-transmitting glass taking the placeof a pair of rods on either side of the core. FIG. 6E shows an IR-PMsolid MOF where birefringence is provided by means of substitution ofthe first IR-transmitting glass in place of two rods adjacent to andcolinear the core. FIG. 6F shows an IR-PM solid MOF where a pair of airholes are adjacent to and on either side of the core.

FIGS. 7A-7E show an IR-PM fiber using anti-resonance or inhibitedcoupling mechanisms. FIG. 7A shows an IR-PM anti-resonant fibercomprising a first IR-transmitting glass and having a claddingcomprising a series of four tube-like structures arranged in a circularconfiguration in a single layer, such that adjacent tubes do not toucheach other and are separated by a space and being attached to an outercladding tube, where the fiber has a hollow core region. FIG. 7B showsan IR-PM anti-resonant fiber with a hollow core region having a diameterdefined by the space between the tubes comprising the cladding, wheretwo of the cladding tubes, B-tubes, colinear with each other and thecore region are different from the other tubes, A-tubes, and where theA-tubes have a different wall thickness than the B-tubes. FIG. 7C showsan IR-PM anti-resonant fiber where the A-tubes comprise a firstIR-transmitting glass, and the B-tubes comprise a second IR-transmittingglass and both sets of tubes have the same wall thickness. FIG. 7D showsan IR-PM anti-resonant fiber where the A-tubes comprise a firstIR-transmitting glass, the B-tubes comprise a second IR-transmittingglass, both sets of tubes have the same wall thickness, and there are anunequal amount of A-tubes and B-tubes. FIG. 7E shows an IR-PManti-resonant fiber where the A-tubes comprise a first IR-transmittingglass, the B-tubes comprise a second IR-transmitting glass, the A-tubeshave a different wall thickness than the B-tubes, and the tubes arenested tubes where an inner tube is joined to an outer tube at the sameattachment point that joins the outer tube to the outer cladding tube.

FIGS. 8A-8B show an IR-PM chiral fiber. FIG. 8A shows a circular IR-PMfiber having a solid core comprising a first IR-transmitting glasssurrounded by a cladding comprising a second IR-transmitting glass, andhaving two stressor members comprising a third glass. FIG. 8B shows howthe stressor members are coiled around the fiber core for the fibershown in FIG. 8A.

FIGS. 9A-9F show an IR fiber with an endface polarizer. FIG. 9A shows anIR fiber having a proximal endface and a distal endface. FIG. 9B showsan IR fiber having an endface with polarizing features. FIG. 9C shows amold with ridges and valleys to be pressed into an IR fiber. FIG. 9Dshows ridges and valleys formed on an endface of an IR fiber bynano-indenting where a mold is pressed into the fiber endface and thenremoved. FIGS. 9E and 9F show the deposition of a conductive metal ontothe ridges of the IR fiber shown in FIG. 9D at an angle such that theridges block part of the valleys thereby precluding deposition on partof the structure. FIG. 9E shows the metal being deposited in a firstdirection, and FIG. 9F shows the metal being deposited a second time ina second direction, which is opposite of the first direction.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention described herein, including the various aspects and/orembodiments thereof, meets the unmet needs of the art, as well asothers, by providing IR-transmitting polarization maintaining (IR-PM)optical fibers, and methods of making them. The fibers of the presentinvention comprise IR-transmitting, non-silica-containing glassesincluding but not limited to chalcogenide glasses comprising at leastone chalcogen element excluding oxygen (S, Se, Te), heavy-metal oxideglasses (based on oxides of Te, Ge, Pb, La or others), fluoride glasses,and any combination thereof.

Fiber and Method 1: IR-PM Fiber with Elliptical Core by Preform Drawing

FIG. 1A shows an IR-PM fiber made using the double crucible process(DCP) to form a cane 110 having a core 112 surrounded by a cladding 114.The cane 110 has a core diameter, D1, between 20 μm and 3 mm, an outerdiameter, D2, between 1 and 3 mm, and a core/clad ratio, or D1/D2. Thecane 110 is subsequently compressed to form a compressed cane 111 withan approximately elliptical transverse cross-section and anapproximately elliptical core 113 (FIG. 1A). The compressed cane 111 isinserted into a jacket tube 118 forming an assembly 119 where there isair space 116 between the compressed cane 111 and the jacket tube 118(FIGS. 1B and 1C). The assembly 119 is subsequently stretched on a fiberoptic drawing tower into a fiber 120 comprising a fiber core 122, afiber cladding 124, an over-cladding 128, and a boundary between thecladding and over-cladding 126 (FIG. 1D). In one embodiment, the cane110 is compressed on the fiber draw tower in-situ with the cane-formingstep wherein a pair of compression rollers (or similar) applies acompressive force to the cane 110 after it exits the crucible. Inanother embodiment, the compression occurs off-line by placing the cane110 collected from the draw tower into a press, heating the press andcane to a temperature, T, where Tg+10<T<Tg+75, where Tg is the glasstransition temperature. The elliptical compressed cane 111 is theninserted into a jacket tube 118 with a hole having an internal diameterclose fitting the outer diameter of the compressed cane 111 forming anassembly 119. In one embodiment, the tube 118 has a round hole slightlylarger than the long axis of the elliptical cane (FIG. 1). In anotherembodiment, the tube 118 has an elliptical hole with a long internaldiameter (LID) and short internal diameter (SID) such that the LID isslightly (<20%) larger than the long outer diameter (LOD) of thecompressed cane 111 and the SID is slightly (<20%) larger than the shortouter diameter (SOD) of the compressed cane 111 (FIG. 1C). The assembly119 is then stretched into a PM fiber using in-situ preformconsolidation technique wherein the assembly 119 is heated at its distalend until it slumps under its own weight forming a tapered region and adrop, a vacuum is applied to the void between the elliptical cane andthe tube, the drop is collected forming a fiber between the drop and theproximal end of the assembly. The fiber is then formed by pulling fromthe assembly 119, while the assembly 119 is simultaneously fed into thehot zone of the fiber drawing furnace and a vacuum is continuouslyapplied. In one embodiment, the cane and tube assembly is fused prior tothe drawing step. In this embodiment, the assembly is placed undervacuum, the distal end of the assembly is heated such that the cane andtube completely fuse at the distal end, subsequently the proximal end ofthe assembly is heated such that the cane and tube completely fuse atthe proximal end sealing a void along the length of the assemblycomprising a vacuum. In some embodiments, this void is consolidated bysubsequently heating the assembly and applying high pressure (about1000-3000 psi). In some embodiments, a compression band comprisingheat-shrink tubing or similar mechanical device is applied to theoutside of the tube on at least one of the proximal end, the distal end,or the entire length of the assembly. Fibers formed from this methodhave birefringence greater than about 10⁻⁵. In one embodiment, a preform130 comprising a core 132 surrounded by a cladding 134 is constructedwith a round transverse cross-sectional shape (FIG. 1E). A compressedpreform 131 is formed by compressing the preform 130 perpendicular tothe preform axis to deform the cross-sectional shape of both thecompressed core 133 and the compressed cladding 135 into anapproximately elliptical shape (FIG. 1F). The cross-sectional shape ofthe compressed cladding 135 is made round by grinding, polishing and/ormachining (FIG. 1G). Then a fiber 140 comprising a fiber core 142surrounded by a fiber cladding 144 is drawn (FIG. 1H) in a mannerconsistent with preform fiber drawing. In some embodiments, the fiber140 comprises at least one chalcogenide glass comprising at least onechalcogen element (S, Se or Te) and other elements including, but notlimited to, Ge, As, Ga, Sn, Sb, Pb, Bi.

Fiber and Method 2: IR-PM Fiber with Elliptical Core Driven by SurfaceTension During Preform Drawing

As shown in FIGS. 2A-2D, an IR-PM fiber 240 having an elliptical core222 surrounded by a round cladding 224 (FIGS. 2B and 2D) is fabricatedby stretching a preform 230 on a fiber optic draw tower to form a fiberwith a group birefringence greater than about 10⁻⁴. The preform 230comprises a core 212, comprising a first IR-transmitting glass andhaving a circular transverse cross-sectional shape, surrounded by acladding 214, comprising a second IR-transmitting glass and having atransverse cross-sectional shape that is not a circle (FIGS. 2A and 2C).In one embodiment, the transverse cross-sectional shape of the cladding214 is elliptical or approximately elliptical (FIG. 2A). In anotherembodiment, the transverse cross-sectional shape of the cladding 214 isapproximately circular, but with two truncated or flattened sides (FIG.2C). During the preform fiber drawing process of the present invention,the preform 230 is held vertically in a fiber drawing oven, such thatthe distal end of the preform is heated until it slumps under its ownweight forming a tapered section and a drop. The drop is then pulled ata controlled rate forming a fiber with a diameter between about 50 μmand 500 μm while the preform 230 is fed into the oven at anothercontrolled rate. The temperature of the preform 230 in the taperedsection is about between 50° C. and 150° C. greater than the Tg of theglass comprising the preform 230, and the viscosity of the glasscomprising the preform is about 10⁴-10⁶ Poise. Surface tension at thisviscosity causes the transverse cross-sectional shape of the fibercladding 224 to become round in the fiber 240 (FIGS. 2B and 2D) therebycausing the fiber core 222, which was round in the preform 230 (FIGS. 2Aand 2C), to become elliptical as glass flows during the fiberization. Insome embodiments, the IR-transmitting glass of the core comprises achalcogenide glass. In some embodiments, the IR-transmitting glass ofthe cladding comprises a chalcogenide glass comprising at least onechalcogen element (S, Se or Te) and other elements including, but notlimited to, Ge, As, Ga, Sn, Sb, Pb, Bi. In one embodiment, at least oneof the core glass or the cladding glass comprises a chalcohalide glass.In one embodiment, at least one of the core glass or the cladding glasscomprises a heavy-metal oxide glass. In one embodiment, at least one ofthe core glass or the cladding glass comprises a fluoride glass. In someembodiments, the preform is fabricated by extruding the secondIR-transmitting glass through a suitable die to form a tube with a roundhole and a non-round outer transverse cross-sectional shape, inserting asolid rod with round transverse cross-sectional shape and comprising thefirst IR-transmitting glass, into the hole in the tube. In someembodiments, the preform is fabricated by traditional rod-in-tubemethods with a round transverse cross-sectional shaped cladding that ismodified by grinding, polishing or machining to have a non-round shape(FIG. 2C), prior to drawing into fiber.

Fiber and Method 3: IR-PM Fiber with Elliptical Core by DCP Drawing

As shown in FIGS. 3A-3F, an IR-PM fiber 340 transmitting infrared lightwith a group birefringence greater than about 10⁻⁴ has a solid core 322with an approximately elliptical transverse cross-sectional shape andcomprising a first IR-transmitting glass and a solid cladding 324comprising a second IR-transmitting glass (FIGS. 3C and 3F). The fiber340 is drawn using the double crucible process (DCP) wherein at leastone of the crucibles has a non-round transverse shaped exit aperture ortip. In one embodiment, the core crucible has a core tip 311 and a corechannel 313 that each have an elliptical transverse cross-sectionalshape with a ratio of long internal diameter to short internal diameter(LID/SID) between 1.5 and 3 (FIG. 3D) to provide an approximatelyelliptical shape to the core glass 312 as it exits the die (FIG. 3E).The clad crucible has a clad tip 315 and a clad channel 317 that eachhave a round transverse cross-sectional shape (FIG. 3D) to provide anapproximately round shape to the clad glass 314 (FIG. 3E). In anotherembodiment, the core tip 311 and core channel 313 each have a roundtransverse cross-sectional shape (FIG. 3A) to provide an approximatelyround shape to the core glass 312 (FIG. 3B). The clad tip 315 and theclad channel 317 each have an elliptical transverse cross-sectionalshape with a ratio of long internal diameter to short internal diameter(LID/SID) between 1.5 and 3 (FIG. 3A) to provide an approximatelyelliptical shape to the clad glass 314 upon exit from the crucible (FIG.3B). Due to surface tension, the clad glass 314 becomes round during thefiber draw and thereby forces the fiber core 322 to become approximatelyelliptical while the fiber cladding 324 is approximately round (FIG.3C). In some embodiments, the fiber comprises at least one chalcogenideglass comprising at least one chalcogen element (S, Se or Te) and otherelements including, but not limited to, Ge, As, Ga, Sn, Sb, Pb, Bi.

Fiber and Method 4: IR-PM Fiber with Stress-Induced Birefringence

An IR-PM fiber 440 transmitting infrared light with a groupbirefringence greater than about 10⁻⁴ has a solid core 422 comprising afirst chalcogenide glass, a solid cladding 424 comprising a secondchalcogenide glass, and at least two solid stressor members 423comprising a third chalcogenide glass (FIG. 4E). The core 422 has adiameter as small as 4 μm and as large as 20 μm and is located entirelywithin the cladding 424 having a diameter of as small as 50 μm and aslarge as 500 μm or larger. The stressors 423 are on either side of thecore 422, each has a diameter as small as 10 μm and as large as 200 μm,and the diameters of each are about the same as each other and entirelywithin the cladding 424. The first chalcogenide glass comprises at leastone chalcogen element (S, Se or Te) and other elements including, butnot limited to, Ge, As, Ga, Sn, Sb, and any combination thereof,transmits infrared light from between about 1 μm to about 12 μm orgreater, depending on composition; and has a coefficient of thermalexpansion, CTE1. The second chalcogenide glass comprises at least onechalcogen element (S, Se or Te) and other elements including, but notlimited to, Ge, As, Ga, Sn, Sb, and any combination thereof; transmitsinfrared light from between about 1 μm to about 12 μm or greater,depending on composition; and has a coefficient of thermal expansion,CTE2. The third chalcogenide glass comprises at least one chalcogenelement (S, Se or Te) and other elements including, but not limited to,Ge, As, Ga, Sn, Sb, and any combination thereof, transmits infraredlight from between about 1 μm to about 12 μm or greater, depending oncomposition; and has a coefficient of thermal expansion, CTE3. Thethermal expansion coefficient of the first and second glasses (CTE1 andCTE2) are between about 20 and 27 ppm/° C. The softening temperatures ofthe three glasses are about the same, such that the glass transitiontemperature (Tg) of each glass is no more than 10° C. higher or lowerthan the other glasses. In one embodiment, the CTE3 is between 2 and 10ppm/° C. lower than CTE2. In another embodiment, the CTE3 is between 4and 10 ppm/° C. higher than CTE2. In one embodiment, the stressormembers 423 have a circular transverse cross-sectional shape. In oneembodiment, the first chalcogenide glass comprises As and S, the secondchalcogenide glass comprises As and S, and the third chalcogenide glasscomprises As, Se, Te and Ge. In one embodiment, the first chalcogenideglass comprises As and S, the second chalcogenide glass comprises As andS, and the third chalcogenide glass comprises at least one halogenelement (I, Br, Cl, or F). In one embodiment, the first chalcogenideglass comprises As and Se, the second chalcogenide glass comprises Asand Se, and the third chalcogenide glass comprises As and S with greaterthan 60 atomic % of S. In some embodiments, the third IR glasscomprising the stressor members 423 has a refractive index smaller thanthose of the first and second IR glasses.

A method of making said fiber 440 wherein three separate parts are firstfabricated: a core cane 412 (FIG. 4A), a cladding preform 414 (FIG. 4B)having multiple openings 415 and 416, and a pair of stressor canes 413(FIG. 4C). The parts are assembled by inserting the core cane 412 andstressor canes 413 into designated openings, 415 and 416 respectively,in the cladding preform 414 to form an assemblage 430 (FIG. 4D). Asshown in FIG. 4D, there may be interstitial space 417 between the canes412, 413 and the cladding preform 414 in the assemblage 430. Theassemblage 430 is stretched into an IR-PM fiber 440 (FIG. 4E). The corecane 412, has a diameter of as small as 100 μm and as large as 2 mm orpossibly larger or smaller, comprises the first chalcogenide glass, andis formed by casting, stretching or extruding the first chalcogenideglass. The cladding preform 414 has a diameter as small as 5 mm and aslarge as 25 mm or possibly larger, comprises a plurality (3 in thisexample) of openings, and is formed by passing the second chalcogenideglass through an extrusion die. The stressor canes 413 comprise thethird chalcogenide glass, have a diameter of as small as 1 mm and aslarge as 12 mm or possibly larger, and are formed by casting, stretchingor extruding the third chalcogenide glass. In one embodiment, theassemblage 430 is fused after assembly but prior to drawing into fiberby means of heating of the assemblage 430. In another embodiment, theassemblage 430 is held together with a fixture and a vacuum is appliedto the interstitial space 417 between the canes 412, 413 and thecladding preform during the fiber draw step. In one embodiment, the corecane 452 comprises an inner core glass 453 and an outer clad glasscoating 454 (FIG. 4F). The inner core glass 453 comprises the firstchalcogenide glass, and the outer clad glass coating 454 comprises thesecond chalcogenide glass. This core cane 452 is fabricated by thesimultaneous flow of two different molten glasses through a cruciblewith concentric openings, the co-extrusion of two different glassesthrough an extrusion die, or by assembling a core rod inside a thincladding tube, and stretching the assemblage into a core cane 452.

Fiber and Method 5: IR-PM MOF

As shown in FIGS. 5A-5E, an IR-PM fiber transmitting infrared light,having a group birefringence greater than 10⁴, comprises a firstIR-transmitting glass, having a solid core, a series of longitudinalair-holes arranged in a lattice having a lattice pitch, Λ, having adiameter D1, and surrounding the core (FIG. 5A shows the preform). Atleast 3 and as many as 10 rings of air holes surround the core. In oneembodiment, the core is birefringent by means of a pair of air-holes oneither side of the core having a diameter D2 larger than the diameter ofthe rest of the holes, D1 (FIG. 5B). In one embodiment, the core isbirefringent by means of all holes on either side of and colinear withthe core having a diameter D2 larger than the diameter of the rest ofthe holes, D1 (FIG. 5C). In one embodiment, the core is birefringent bymeans of a second IR-transmitting glass 545 having a refractive indexlower than the first IR-transmitting glass taking the place of a pair ofair holes on opposing sides of the core (FIG. 5D). In some embodiments,birefringence is provided by means of substitution of the firstIR-transmitting glass in place of at least 1 air holes adjacent to andcolinear the core (FIG. 5E). In some embodiments, the fiber comprises atleast one chalcogenide glass comprising at least one chalcogen element(S, Se or Te) and other elements including, but not limited to, Ge, As,Ga, Sn, Sb, Pb, Bi, and any combination thereof.

A method of fabricating the IR-PM fiber transmitting infrared light andhaving a group birefringence greater than 10⁻⁴, where the fiber is drawnon a fiber optic draw tower from a preform comprising a firstIR-transmitting glass and having multiple openings. In some embodiments,the preform is fabricated by extruding the first IR-transmitting glassthrough a suitable die. In some embodiments, the preform is fabricatedby drilling or mechanically abrading a plurality of holes in a solid rodcomprising the first IR-transmitting glass. In some embodiments, atleast 1 cane comprising a second IR-transmitting glass is inserted intoat least 1 hole adjacent to the core in the preform prior to fiberdrawing. In some embodiments, gas pressure is applied to at least 1opening in the preform during the fiber draw process to maintain theratio of hole diameter to hole pitch, D1/A, or to increase it comparedto the ratio in the preform. In some embodiments, a first gas pressureis applied to the holes adjacent to the core and a second gas pressureis applied to the other holes. In this method, gas pressure may bepositive or negative (e.g. applied by a vacuum) and may constitutenitrogen, argon, helium, some other inert or reactive gas, or anycombination thereof.

Fiber and Method 6: IR-PM Solid MOF

As shown in FIGS. 6A-6F, an IR-PM fiber transmitting infrared light,having a group birefringence greater than 10⁻⁴, comprises a firstIR-transmitting glass 635 having a solid core and a series oflongitudinal solid rods comprising a second IR-transmitting glass 637having a refractive index different from that of the first glass,arranged in a lattice having a lattice pitch, Λ, having a diameter D1,and surrounding the core (FIG. 6A shows the preform). At least 3 and asmany as 10 rings of rods surround the core. In one embodiment, the coreis birefringent by means of a pair of rods on either side of the corehaving a diameter D2 larger than the diameter of the rest of the rods,D1 (FIG. 6B). In one embodiment, the core is birefringent by means ofall rods on either side of and colinear with the core having a diameterD2 larger than the diameter of the rest of the rods, D1 (FIG. 6C). Inone embodiment the core is birefringent by means of a thirdIR-transmitting glass 639 having a refractive index lower than the firstIR-transmitting glass taking the place of a pair of rods on either sideof the core and having a diameter D2 that may be equal to, larger thanor smaller than the diameters of the remaining rods, D1 (FIG. 6D). Insome embodiments, birefringence is provided by means of substitution ofthe first IR-transmitting glass in place of at least 1 rod adjacent toand colinear the core (FIG. 6E). In another embodiment, a pair of airholes 641 are adjacent to and on opposite sides of the core (FIG. 6F).In some embodiments, the fiber comprises at least one chalcogenide glasscomprising at least one chalcogen element (S, Se or Te) and otherelements including, but not limited to, Ge, As, Ga, Sn, Sb, Pb, Bi, orany combination thereof.

A method of fabricating the IR-PM fiber transmitting infrared light andhaving a group birefringence greater than 10⁻⁴, wherein the fiber isdrawn on a fiber optic draw tower from a preform comprising a firstIR-transmitting glass and a second IR-transmitting glass. In someembodiments, the preform is fabricated by extruding the firstIR-transmitting glass through a suitable die such that the preformcontains a plurality of longitudinal holes, into which a series of rodscomprising the second IR-transmitting glass are inserted. In someembodiments, the preform is fabricated by drilling or mechanicallyabrading a plurality of holes in a solid rod comprising the firstIR-transmitting glass and a series of rods comprising the secondIR-transmitting glass are inserted. In some embodiments, at least 1 canecomprising a third IR-transmitting glass is inserted into at least 1hole adjacent to the core in the preform prior to fiber drawing. In someembodiments, some of the holes do not contain a rod and gas pressure isapplied to at least one opening in the preform during the fiber drawprocess to maintain the ratio of hole diameter to hole pitch, D1/A, orto increase it compared to the ratio in the preform. In this method, gaspressure may be positive or negative (e.g. applied by a vacuum) and mayconstitute nitrogen, argon, helium, some other inert or reactive gas, orany combination thereof.

Fiber and Method 7: IR-PM Anti-Resonant Fiber

FIGS. 7A-7E show an IR-PM fiber transmitting infrared light usinganti-resonance or inhibited coupling mechanisms, having a groupbirefringence greater than about 10⁻⁴, comprising a firstIR-transmitting glass and having a cladding comprising a series of atleast four and as many as ten tube-like structures 702, 703, 704, 705(FIG. 7A) each having a wall thickness, t, and an outer diameter, d,being arranged in a circular configuration in a single layer, such thatadjacent tubes do not touch each other and are separated by a space, s,and being attached to an outer cladding tube 701 (FIG. 7A). The fiberhas a hollow core region 706 (FIG. 7A) with diameter d_(core) defined bythe space between the tube-like structures. In some embodiments, thefiber is birefringent due to differences in at least two of thetube-like structures surrounding the hollow core region 706, wherein oneset of the tube-like structures, termed B-tubes, colinear with eachother and the core region 706 are different from the other tube-likestructures, termed A-tubes. In some embodiments, the A-tubes 717comprise a first IR-transmitting glass and the B-tubes 718 comprise asecond IR-transmitting glass having a refractive index different fromthat of the first IR-transmitting glass (FIG. 7C). In some embodiments,the A-tubes 714 and B-tubes 715 comprise the same IR-transmitting glass,but the A-tubes have a different wall thickness (t_(A)) than the B-tubes(t_(B)), (t_(A)≠t_(B)) (FIG. 7B). In some embodiments, the A-tubes 717comprise a first IR-transmitting glass, and the B-tubes 718 comprise asecond IR-transmitting glass and both sets of tubes have the same wallthickness (t_(A)=t_(B)) (FIG. 7C). In some embodiments, the A-tubes 719comprise a first IR-transmitting glass, the B-tubes 720 comprise asecond IR-transmitting glass, both sets of tubes have the same wallthickness, and there are an unequal amount of A-tubes and B-tubes (e.g.4 A-tubes and 2 B-tubes) (FIG. 7D). In some embodiments, the A-tubescomprise a first IR-transmitting glass with a refractive index n_(A)(λ)and the B-tubes comprise a second IR-transmitting glass with a secondrefractive index n_(B)(λ), such that n_(A)(λ)≠n_(B)(λ) and the A-tubeshave a different wall thickness (t_(A)) than the B-tubes (t_(B)), suchthat (t_(A)≠t_(B)). In some embodiments, the tube-like structures 721,722 are nested tubes wherein an inner tube with a smaller diameter,d_(A2) or d_(B2) for A-tube 721 or B-tube 722 positions respectively, isjoined to an outer tube with a larger diameter, d_(A1) or d_(B1) forA-tube 721 or B-tube 722 sites respectively, at the same attachmentpoint that joins the outer tube to the outer cladding tube as shown inFIG. 7E where the tube thicknesses are t_(A1), t_(A2), t_(B1) and t_(B2)for the large A-tubes, small A-tubes, large B-tubes and small B-tubesrespectively. In some embodiments, t_(A1)=t_(A2), t_(B1)=t_(B2),d_(A1)=d_(B1), d_(A2)=d_(B2), and t_(B1)>t_(A1). In some embodiments,t_(A1)=t_(A2), t_(B1)=t_(B2), d_(A1)=d_(B1), d_(A2)=d_(B2), andt_(B1)<t_(A1). In some embodiments, the A-tubes comprise a firstIR-transmitting glass with a refractive index n_(A)(λ) and the B-tubescomprise a second IR-transmitting glass with a second refractive indexn_(B)(λ), t_(A1)=t_(A2)=t_(B1)=t_(B2), d_(A1)=d_(B1)=d_(A2)=d_(B2), andn_(A)(λ)≠n_(B)(λ). Although the example shown in FIG. 7E shows fourtotal tube positions, two A-tube positions 721 and two B-tube positions722, the number of positions may be greater, for example ten where eightare A-tube positions and two are B-tube positions or six are A-tubepositions and four are B-tube positions. In some embodiments, the firstIR-transmitting glass comprises a first chalcogenide glass consisting ofat least one element from the group of sulfur, selenium and telluriumand at least one other element, such as but not limited to, arsenic,germanium, gallium, antimony, and any combination thereof, and thesecond IR-transmitting glass comprises a second chalcogenide glassconsisting of at least one element from the group of sulfur, seleniumand tellurium and at least one other element, such as but not limitedto, arsenic, germanium, gallium, antimony and any combination thereof.

A method of fabricating an IR-PM fiber wherein a preform having the samefeatures as desired in the fiber, but larger in scale and perhaps withdifferent dimensional proportions or ratios, is drawn on a fiber opticdraw tower at an elevated temperature. In some embodiments of themethod, the preform is fabricated by extruding an IR-transmitting glassthrough a suitable die at an elevated temperature. In some embodiments,at least one gas pressure is applied to at least one of the openings inthe preform using an inert gas such as nitrogen, helium, argon, or othergas or a vacuum. In some embodiments of the method, at least twodifferent IR-transmitting glasses are co-extruded through a suitable dieat an elevated temperature to produce a preform having tubes withdifferent glass compositions on the A- and B-tube positions. In someembodiments of the method, the preform is fabricated by assemblingsuitable tubes, each comprising a suitable IR-transmitting glass, in asuitable arrangement such that the tubes contact an outer cladding tubeand are separated from each other by means of a suitable jig that may bepresent at one or both ends of the assemblage or assembled preform. Insome embodiments, the jig comprises a material including but not limitedto a glass, a polymer, an epoxy, or a metal. In some embodiments, atleast part of the assembled preform is heated, prior to drawing intofiber, to fuse the individual tubes to the outer cladding tube.

Fiber and Method 8: IR-PM Chiral or Twisted Fiber

FIGS. 8A and 8B show a circular-polarization maintaining IR-transmittingoptical fiber 840 having a solid core 842 comprising a firstIR-transmitting glass surrounded by a cladding 844 comprising a secondIR-transmitting glass having a refractive index lower than that of thefirst glass, and having at least one stressor member 813 comprising athird glass. The stressor member(s) 813 run the entire length of theoptical fiber 840, are coiled around the solid core 842, and arepositioned close to the core 842 such that the core 842 has astress-induced birefringence oriented within the transverse plane, in adirection defined by the location of the stressor member(s) 813 relativeto the core as shown in FIG. 8A. Since the stressor member(s) 813 arecoiled around the fiber core 842 (FIG. 8B), the direction of thebirefringence vector, along the length of the fiber, rotates around thecore but is confined to the transverse plane such that the fiber 840transmits and maintains the circular polarization of the lightintroduced to it. In one embodiment, the circular-polarizationmaintaining IR fiber 840 has one stressor member 813, adjacent to andcoiled around the fiber core 842. In another embodiment, thecircular-polarization maintaining IR fiber 840 has two stressor members813 adjacent to and coiled around the fiber core 842 with a common pitch846 and coiling direction (e.g. left-hand wound) such that at anytransverse cross section, the fiber core 842 and both stressor members813 are collinear (FIG. 8B). In some embodiments, the firstIR-transmitting glass is a chalcogenide glass comprising at least onechalcogen element (S, Se or Te) and other elements including, but notlimited to, Ge, As, Ga, Sn, Sb, Pb, Bi, and any combination thereof. Thesecond IR-transmitting glass is a chalcogenide glass comprising at leastone chalcogen element (S, Se or Te) and other elements including, butnot limited to, Ge, As, Ga, Sn, Sb, Pb, Bi, and any combination thereof.In some embodiments, the third glass transmits IR light and has arefractive index lower than that of the second glass.

A method to fabricate a circular-polarization maintainingIR-transmitting optical fiber wherein a preform having a solid corecomprising a first IR-transmitting glass surrounded by a claddingcomprising a second IR-transmitting glass having a refractive indexlower than that of the first glass, and having at least one stressormember comprising a third glass is drawn into an optical fiber on afiber optic draw tower using the preform draw method. The preform isrotated about its axis throughout the drawing. In some embodiments, thepreform is rotated at a constant rate. In some embodiments the preformis fabricated by drilling a core hole and at least one other hole in arod comprising the second glass and inserting a cane of the first glassinto the core hole and a cane comprising the third glass into each ofthe other holes, and then heating the assemblage to fuse the rods andcanes. In some embodiments, a vacuum is used to remove voids during thepreform fusing step.

Fiber and Method 9: IR Fiber with Endface Polarizer

FIGS. 9A-9F show an infrared-transmitting fiber 920 having a solid core905 (FIG. 9B) comprising at least one IR-transmitting material, aproximal endface 902 and a distal endface 904 (FIG. 9A), where at leastone of said endfaces 902, 904 incorporates a polarizing structure 906comprising at least one conductive material. In some embodiments, thepolarizing structure 906 comprises a series of features such as linearridges and valleys, and a metal 942, such as aluminum, applied to atleast part of the ridges and/or valleys forming metallic lines, and saidfeatures are arranged in a repeating periodic pattern having a spacingor pitch 946 between adjacent features (FIG. 9B). In the example shownin FIG. 9B, the features cover about 50% of the endface including thecore 905. The spacing or pitch 946 is narrower than the wavelength oflight being transmitted by the fiber, and in some embodiments the pitch946 is about 1000 nm. The height of the ridges 948 relative to thebottom of the valleys is in the range 100-1000 nm or larger depending onthe wavelength of light being transmitted by the fiber. The thickness ofthe metal features is in the range of 20-100 nm or larger, and the width947 is typically about 20-50% of the pitch, but may vary from thosesizes. The features on the fiber endface, particularly the conductivemetal lines, allow the transmission of light polarized in the directionperpendicular to the direction of the metal lines and suppress lightpolarized in the direction parallel to the lines with a large extinctionratio (about 10 dB or greater). In some embodiments, the polarizingstructure is on one endface but not the other; in other embodiments,both endfaces have the polarizing structure. In some embodiments, thefiber comprises an IR-transmitting glass, such as a chalcogenide glasscomprising at least one chalcogen element (S, Se or Te) and otherelements including, but not limited to, Ge, As, Ga, Sn, Sb, Pb, Bi, orany combination thereof. In some embodiments, the fiber comprises asolid cladding. In some embodiments, the fiber has other polarizationmaintaining features such as those described elsewhere in thisdisclosure, and the polarizing endface features are in a directioncompatible with the polarization maintaining features in the fiber. Insome embodiments, the polarizing endface features are part of a cap orseparate part that has been affixed to the endface of the fiber, whichin said embodiments may have hollow light guiding cores instead of solidcores.

A method to fabricate an infrared-transmitting optical fiber having apolarizing structure on at least one fiber endface, has the followingsteps: (1) Ridges and valleys are formed on the endface of the fiber 920either by nano-indenting, where a mold 960 or shim having a negativerelief of the desired features is pressed into the fiber endfacedeforming it into the desired texture (FIGS. 9C and 9D), or by anetching process such as reactive ion etching. (2) A conductive metal,such as aluminum, gold, silver, platinum or other, is deposited using aline-of-sight deposition process including, but not limited tosputtering, thermal evaporation, or another method. In some embodiments,the deposition is performed at an angle such that the ridges block partof the valleys thereby precluding deposition on part of the structure asshown in FIG. 9E. The metal may be deposited in one direction first(FIG. 9E) and then deposited a second time in the opposite direction(FIG. 9F) to make wider metal features, although this is not arequirement of the invention. In some embodiments, the ridges andvalleys have square or approximately square cross-sectional shapes. Inother embodiments, the ridges and valleys have sharp or approximatelytriangular cross-sectional shapes as shown in FIGS. 9B-9F. Sharpness ofthe indented features is not a requirement of the method.

In another embodiment, the first step of indenting the fiber endfaceusing a mold is omitted and a shadow mask is used to form the patternduring the deposition step.

Throughout this application, various patents and publications have beencited. The disclosures of these patents and publications in theirentireties are hereby incorporated by reference into this application,in order to more fully describe the state of the art to which thisinvention pertains.

The invention is capable of modification, alteration, and equivalents inform and function, as will occur to those ordinarily skilled in thepertinent arts having the benefit of this disclosure. While the presentinvention has been described with respect to what are presentlyconsidered the preferred embodiments, the invention is not so limited.To the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the description provided above.

What is claimed:
 1. A method of making infrared-transmitting,polarization maintaining (IR-PM) optical fiber, comprising: forming acane comprising an infrared-transmitting, non-silica glass; compressingthe cane to form a compressed cane with an approximately ellipticalcross-section and an approximately elliptical core; inserting thecompressed cane into a tube to form an assembly; and stretching theassembly into a fiber that is approximately round with an approximatelyelliptical core, wherein the fiber has a birefringence greater than10⁻⁵.
 2. The method of claim 1, wherein the infrared-transmitting,non-silica glass comprises a chalcogenide glass comprising at least oneof sulfur, selenium, and tellurium; a heavy metal oxide glass; afluoride glass, or any combination thereof.
 3. A method of making aninfrared-transmitting, polarization maintaining (IR-PM) optical fiber,comprising: making a preform with an approximately round cross-sectionalshape comprising an approximately round core surrounded by a cladding,wherein the preform comprises an infrared-transmitting, non-silicaglass; compressing the preform to form a compressed preform where boththe core and cladding have an approximately elliptical shape; alteringthe compressed preform by grinding, polishing, machining, or anycombination thereof to make an altered preform with an approximatelyround cladding; and drawing the altered preform into a fiber that isapproximately round with an approximately elliptical core, wherein thefiber has a birefringence greater than 10⁻⁵.
 4. The method of claim 3,wherein the infrared-transmitting, non-silica glass comprises achalcogenide glass comprising at least one of sulfur, selenium, andtellurium; a heavy metal oxide glass; a fluoride glass, or anycombination thereof.
 5. A method of making an infrared-transmitting,polarization maintaining (IR-PM) optical fiber, comprising: making apreform comprising a preform glass that comprises a core having anapproximately circular cross-sectional shape and comprising a firstinfrared-transmitting, non-silica glass and a cladding surrounding thecore, wherein the cladding comprises a second infrared-transmitting,non-silica glass and has a cross-sectional shape that is not a circle;and drawing the preform into a fiber that is approximately round with anapproximately elliptical core, wherein the fiber has a birefringencegreater than 10⁻⁵, wherein during drawing the preform is held verticallyand heated in a fiber drawing oven until one end of the preform slumpsand forms a tapered section and a drop, wherein the drop is pulled at acontrolled rate, wherein the temperature of the preform in the taperedsection is between 50 and 150° C. greater than the glass transitiontemperature of the preform glass, and wherein the viscosity of thepreform glass is 10⁴-10⁶ Poise.
 6. The method of claim 5, wherein thefirst infrared-transmitting, non-silica glass, the secondinfrared-transmitting, non-silica glass, or both comprise a chalcogenideglass comprising at least one of sulfur, selenium, and tellurium; aheavy metal oxide glass; a fluoride glass, or any combination thereof.7. A method of making an infrared-transmitting, polarization maintaining(IR-PM) optical fiber, comprising: providing a double cruciblecomprising a core crucible and a clad crucible, wherein at least one ofthe core and clad crucibles comprises an approximately elliptical shapedcross-sectional exit aperture; loading a first infrared-transmitting,non-silica glass into the core crucible; loading a secondinfrared-transmitting, non-silica glass into the clad crucible; forminga glass comprising a core surrounded by a cladding; and drawing a fiberthat is approximately round with an approximately elliptical core,wherein the fiber has a birefringence greater than 10⁻⁴.
 8. The methodof claim 7, wherein the first infrared-transmitting, non-silica glass,the second infrared-transmitting, non-silica glass, or both comprise achalcogenide glass comprising at least one of sulfur, selenium, andtellurium; a heavy metal oxide glass; a fluoride glass, or anycombination thereof.
 9. A method of making an infrared-transmitting,polarization maintaining (IR-PM) optical fiber, comprising: making acore cane comprising a first infrared-transmitting, non-silica glass;making a cladding preform comprising a second infrared-transmitting,non-silica glass, wherein the cladding preform comprises a plurality ofopenings; making at least two stressor canes comprising a thirdinfrared-transmitting, non-silica glass; forming an assemblagecomprising the core cane and stressor canes inserted into the pluralityof openings in the cladding preform; and stretching the assemblage intoa fiber, wherein the fiber has a birefringence greater than 10⁻⁴. 10.The method of claim 9, wherein the first infrared-transmitting,non-silica glass, the second infrared-transmitting, non-silica glass,the third infrared-transmitting, non-silica glass, or any combinationthereof comprises a chalcogenide glass comprising at least one ofsulfur, selenium, and tellurium; a heavy metal oxide glass; a fluorideglass, or any combination thereof.
 11. The method of claim 9, whereinthe core cane comprises an inner core glass and an outer clad glass. 12.The method of claim 11, wherein the inner core glass comprises the firstinfrared-transmitting, non-silica glass and the outer clad glasscomprises the second infrared-transmitting, non-silica glass.
 13. Amethod of making an infrared-transmitting, polarization maintaining(IR-PM) micro-structured optical fiber (MOF), comprising: making apreform comprising an infrared-transmitting, non-silica glass, whereinthe preform has a solid core and a plurality of openings; and drawingthe preform into a fiber comprising a solid core and a series oflongitudinal air-holes arranged in a lattice, wherein the fiber has agroup birefringence greater than 10⁻⁴.
 14. The method of claim 13,wherein the infrared-transmitting, non-silica glass comprises achalcogenide glass comprising at least one of sulfur, selenium, andtellurium; a heavy metal oxide glass; a fluoride glass, or anycombination thereof.
 15. The method of claim 13, wherein a pair ofair-holes on opposite sides of the core have a diameter larger than thediameter of the rest of the air-holes.
 16. The method of claim 13,wherein all of the air-holes on either side of and colinear with thecore have a diameter larger than the diameter of the rest of theair-holes.
 17. The method of claim 13, wherein a secondinfrared-transmitting, non-silica glass replaces a pair of air-holes onopposite sides of the core.
 18. The method of claim 13, wherein theIR-transmitting glass replaces at least one air hole adjacent to thecore.
 19. A method of making an infrared-transmitting, polarizationmaintaining (IR-PM) micro-structured optical fiber (MOF), comprising:forming a preform comprising a first infrared-transmitting, non-silicaglass, wherein the preform has a solid core; and drawing the preforminto a fiber comprising a solid core and a series of longitudinal solidrods comprising a second infrared-transmitting, non-silica glass havinga different refractive index from the first infrared-transmitting,non-silica glass, wherein the series of longitudinal solid rods arearranged in a lattice, and wherein the fiber has a group birefringencegreater than 10⁴.
 20. The method of claim 19, wherein the firstinfrared-transmitting, non-silica glass, the secondinfrared-transmitting, non-silica glass, or both comprise a chalcogenideglass comprising at least one of sulfur, selenium, and tellurium; aheavy metal oxide glass; a fluoride glass, or any combination thereof.21. The method of claim 19, wherein a pair of solid rods on oppositesides of the solid core have a diameter larger than the diameter of therest of the solid rods.
 22. The method of claim 19, wherein all of thesolid rods on either side of and collinear with the solid core have adiameter larger than the diameter of the rest of the solid rods.
 23. Themethod of claim 19, wherein a third infrared-transmitting, non-silicaglass replaces a pair of solid rods on opposite sides of the solid core.24. The method of claim 19, wherein the first IR-transmitting glassreplaces at least one solid rod adjacent to the solid core.
 25. Themethod of claim 19, additionally comprising a pair of air holes adjacentto and on opposite sides of the solid core.
 26. A method of making aninfrared-transmitting, polarization maintaining (IR-PM) anti-resonantfiber, comprising: fabricating a preform comprising aninfrared-transmitting, non-silica glass, wherein the preform has ahollow core region and a cladding comprising a cladding tube attached toat least four and as many as ten tube-like structures arranged in acircular configuration in a single layer such that adjacent tubes do nottouch each other, and wherein two of the tube-like structures areB-tubes that are collinear with each other and the core region and aredifferent from the other tube-like structures that are A-tubes; anddrawing the preform into a fiber, wherein the fiber has a groupbirefringence greater than 10⁻⁴.
 27. The method of claim 26, wherein theinfrared-transmitting, non-silica glass comprises a chalcogenide glasscomprising at least one of sulfur, selenium, and tellurium; a heavymetal oxide glass; a fluoride glass, or any combination thereof.
 28. Themethod of claim 26, wherein the A-tubes comprise a firstinfrared-transmitting, non-silica glass and the B-tubes comprise asecond infrared-transmitting, non-silica glass having a refractive indexdifferent from that of the first infrared-transmitting, non-silicaglass.
 29. The method of claim 26, wherein the A-tubes have a differentwall thickness than the B-tubes.
 30. The method of claim 26, wherein thetube-like structures are nested tubes comprising an inner tube with asmaller diameter joined to an outer tube with a larger diameter at anattachment point that joins the outer tube to the cladding tube.
 31. Amethod of making an infrared-transmitting, polarization maintaining(IR-PM) chiral fiber, comprising: making a preform comprising a solidcore comprising a first infrared-transmitting, non-silica glasssurrounded by a cladding comprising a second infrared-transmitting,non-silica glass having a refractive index lower than that of the firstinfrared-transmitting, non-silica glass, and at least one stressormember comprising a third glass; and drawing the preform into an IR-PMchiral fiber, wherein the preform is rotated about its axis during thedrawing so the at least one stressor member coils around the solid core,and wherein the fiber has a group birefringence greater than 10⁻⁴. 32.The method of claim 31, wherein the first infrared-transmitting,non-silica glass, the second infrared-transmitting, non-silica glass, orboth comprise a chalcogenide glass comprising at least one of sulfur,selenium, and tellurium; a heavy metal oxide glass; a fluoride glass, orany combination thereof.
 33. A method of making aninfrared-transmitting, polarization maintaining (IR-PM) fiber,comprising: making a fiber comprising a solid core comprising aninfrared-transmitting, non-silica glass; forming ridges and valleys onan endface of the fiber; and depositing a conductive metal on the ridgesto form an IR-PM fiber, wherein the fiber has a group birefringencegreater than 10⁻⁴.
 34. The method of claim 33, wherein theinfrared-transmitting, non-silica glass comprises a chalcogenide glasscomprising at least one of sulfur, selenium, and tellurium; a heavymetal oxide glass; a fluoride glass, or any combination thereof.
 35. Themethod of claim 33, wherein the conductive metal comprises aluminum,gold, silver, platinum, or any combination thereof.